Hydrocarbon fuel reformers which are used in fuel cell power plants take raw hydrocarbon fuel such as natural gas, propane or naphtha and catalytically convert the raw fuel to a hydrogen rich fuel gas suitable for use as an anode fuel. Raw fuel is percolated through catalyst beds which are contained in tubes mounted within the reformer housing. Catalytic conversion is carried out at elevated temperatures in the range of 430 C. to 930 C. The reformer includes a pressure vessel as the reformer housing within which a burner is operated to establish these elevated temperatures. Reformer tubes are in the form of bayonet tubes with an upward flow in the outer portion through the catalyst bed and a downward flow through the inner portion providing some counterflow heat recovery from the reformed gas to the incoming gas. The flue gas passes a heat exchange relationship with the outer surface of the bayonet tubes. It is maintained in close contact therewith by surrounding sleeves.
At the lower end of the bayonet tubes there is provided a support and seal plate which supports the sleeves, and seals against a central burner assembly, the pressure vessel, and the sleeves while permitting the gas flowing inside the sleeves to pass through the seal plate to a plenum therebelow.
Because of the high temperatures existing, the seal plate must be of austenitic materials while the pressure vessel is of ferritic material. The pressure vessel is internally insulated and at operating conditions has a temperature level of about 150 C. The internal support and seal plate is exposed to the high temperature gases and accordingly is of austenitic material and operates at about 600 C. Considerable differential expansion exists because of the differential temperature between the seal plate and the pressure vessel, and this differential expansion is exacerbated by the fact that stainless steel has a significantly higher coefficient of expansion than the ferritic material.
Different sealing problems occur at three points with cost and reliability being a factor in each situation.
It is desired that all of the gas flow through the annular space between the sleeve and the bayonet tubes. Accordingly, the seal plate must be sealed to the central burner structure, each of the sleeves, and the pressure vessel.